1. Field of the Invention
The present invention generally relates to perylene diester chromophores which are useful as fluorescent dyes in various applications, including in wavelength conversion films. Such wavelength conversion films can significantly enhance the solar harvesting efficiency of photovoltaic or solar cell devices.
2. Description of the Related Art
In recent years, with the need for new optical light collection systems, fluorescence-based solar collectors, fluorescence-activated displays, and single-molecule spectroscopy, various approaches for preparing perylene dyes have been explored. However, many technical issues have yet to be overcome.
Several papers describe perylene backbone structure chromophores having good wavelength conversion capability. Typically, these chromophores emit longer wavelength fluorescence light upon illumination with shorter wavelength light. For example, Zhang et al., “Synthesis and characterization of perylene tetracarboxylic bisester monoimide derivatives,” Dyes and Pigments Journal, 2008, vol. 76, pp. 810-816 reported n-(1-butyl)perylene-3,4,9,10-tetracarboxylic-3,4-(bis-alkylester)-9,10-imide (C4-Cn) and 1,3-bis[(n-(1-butyl)perylene-3,4:9,10-tetracarboxylic-3-alkylester-9,10-imide)-4-ester]dioxypropane (C4-C3-C4), which show good fluorescence behavior and long stoke-shift wavelength values. Additionally, Jones et al., “Tuning Orbital Energetics in Arylene Diimide Semiconductors. Materials Design for Ambient Stability of n-Type Charge Transport,” J. Am. Chem. Soc., 2007, 129, pp. 15259-15278, described perylene bisimide derivatives synthesized from perylene dianhydrides, which also showed good fluorescence behavior.
Other references have been disclosed regarding perylene derivatives structures and synthetic procedures, along with their areas of application. For example, see U.S. Patent Application Publication Nos. 2008/0087878 and 2008/0114170, and U.S. Pat. Nos. 5,808,073, 6,136,976, 5,472,494, 6,063,181, 6,184,378, 6,326,494, 6,806,368, 6,986,811, 4,262,851, 4,379,934, 4,419,427, 4,446,324, 4,450,273, 4,618,694, 4,667,036, 4,725,690, and 4,845,223, which were disclosed and filed by BASF. Also see U.S. Pat. Nos. 5,077,161 and 5,645,965, were also disclosed and filed by Xerox corp. Also see U.S. Pat. Nos. 5,693,808, 5,874,580, 5,981,773, and 6,166,210, which were disclosed and filed from Chiba Specialty Chemicals Co. Also see U.S. Pat. Nos. 5,019,473, 5,141,837, 5,028,504, 4,746,741, and 4,968,571, which were disclosed from Eastman Kodak Co. Also see U.S. Pat. Nos. 5,123,966, 5,248,774, 5,154,770, 5,264,034, 5,466,807, 4,431,808, 4,501,906, 4,709,029, 4,594,420, and 4,831,140, which were disclosed and filed from Hoechest. Each of the references and patents disclosed herein is hereby incorporated by reference in its entirety.
These references describe various tetra-carboxylic and di-carbonyl perylene derivatives, which include free acid, ester, amide, and imide groups. Most of the di-carbonyl derivatives have two carbonyl groups, which are attached to a peripheral position, such as the 3- and 4-positions of the perylene rings. However, only limited examples disclose para-position di-carbonyl perylene derivatives which contain two carbonyl groups in the para-position of the perylene structure, e.g. the 3- and 9-(or 10-) position of the perylene ring. Examples of di-carbonyl para-position perylene derivative structure were disclosed in U.S. Pat. No. 4,618,694; however, the disclosure was limited to 3- & 9-position di-carbonyl para-position perylene derivatives, in which a cyano group is attached to the 4-position of the perylene.
One of the useful properties of fluorescent (or photo-luminescent) dyes is their ability to absorb a light photon of a particular wavelength, and re-emit the photon at a different wavelength. This phenomenon makes them useful in the photovoltaic industry. The utilization of solar energy offers a promising alternative energy source to the traditional fossil fuels, and therefore, the development of devices that can convert solar energy into electricity, such as photovoltaic devices (also known as solar cells), has drawn significant attention in recent years.
Several different types of mature photovoltaic devices have been developed, including a silicon based device, a III-V and II-VI PN junction device, a Copper-Indium-Gallium-Selenium/Diselenide (CIGS) thin film device, an organic sensitizer device, an organic thin film device, and a Cadmium Sulfide/Cadmium Telluride (CdS/CdTe) thin film device. More detail on these devices can be found in the literature, such as Lin et al., “High Photoelectric Conversion Efficiency of Metal Phthalocyanine/Fullerene Heterojunction Photovoltaic Device” (International Journal of Molecular Sciences, vol. 12, pp. 476, 2011), the contents of which are hereby incorporated by reference. However, the photoelectric conversion efficiency of these devices can still be improved and the development of techniques for such improvement has been an ongoing challenge for many researchers.
One technique developed to improve the efficiency of photovoltaic devices is to utilize a wavelength down-shifting film. Many of the photovoltaic devices are unable to effectively utilize the entire spectrum of light as the materials on the device absorb certain wavelengths of light (typically the shorter UV wavelengths) instead of allowing the light to pass through to the photoconductive material layer where it is converted into electricity. Application of a wavelength down-shifting film absorbs the shorter wavelength photons and re-emits them at more favorable longer wavelengths, which can then be absorbed by the photoconductive layer in the device, and converted into electricity.
This phenomenon is observed in the thin film CdS/CdTe and CIGS solar cells which both use CdS as the window layer. The low cost and high efficiency of these thin film solar cells has drawn significant attention in recent years, with typical commercial cells having photoelectric conversion efficiencies of 10-16%. However, one issue with these devices is the energy gap of CdS, approximately 2.41 eV, which causes light at wavelengths below 514 nm to be absorbed by the CdS instead of passing through to the photoconductive layer where it can be converted into energy. This inability to utilize the entire spectrum of light effectively reduces the overall photoelectric conversion efficiency of the device.
There are three principal approaches to achieve a more efficient utilization of the short wavelength solar spectrum in the CdS/CdTe devices which have been described in the literature, for example see Klampaftis et al. in “Enhancing the performance of solar cells via luminescent down-shifting of the incident spectrum: A review” (Solar Energy Materials and Solar Cells, vol. 93, pp. 1182-1194, 2009). The first approach is to reduce the absorption loss by reducing the CdS layer thickness. However, this approach negatively affects the lifetime and performance of the device. The second approach is to replace the CdS materials with wider band gap materials, such as ZnSe or ZnTe. However, these materials are more expensive and difficult to use. The third approach is to utilize a wavelength down-shifting material.
There have been numerous reports disclosing the utilization of a wavelength down-shifting material to improve the performance of photovoltaic devices. For example, U.S. Patent Application Publication No. 2009/0151785 discloses a silicon based solar cell device which contains a wavelength down-shifting inorganic phosphor material. U.S. Patent Application Publication No. US 2011/0011455 discloses an integrated solar cell comprising a plasmonic layer, a wavelength conversion layer, and a photovoltaic layer. U.S. Pat. No. 7,791,157 discloses a solar cell with a wavelength conversion layer containing a quantum dot compound. U.S. Patent Application Publication No. 2010/0294339 discloses an integrated photovoltaic device containing a luminescent down-shifting material, however no example embodiments were constructed. U.S. Patent Application Publication No. 2010/0012183 discloses a thin film solar cell with a wavelength down-shifting photo-luminescent medium; however, no examples are provided. Each of these patents and patent application publications, which are incorporated herein by reference in their entirety, specifically promote the use of an inorganic material to enable the wavelength down-shifting.
While there have been numerous disclosures of wavelength down-shifting inorganic mediums used in photovoltaic and solar cell devices, there has been very little work reported on the use of photo-luminescent organic mediums for efficiency improvements in photovoltaic devices. The use of an organic medium, as opposed to an inorganic medium, is attractive in that organic materials are typically cheaper and easier to use, making them a better economical choice. However, the poor photostability of the organic luminescent dyes has inhibited their development. Some theoretical modeling and/or simulation of luminescent films applied to CdS/CdTe solar cells is described in the following literature: U.S. Patent Application Publication No. 2010/0186801; B. S. Richards and K. R. McIntosh in “Overcoming the Poor Short Wavelength Spectral Response of CdS/CdTe Photovoltaic Modules via Luminescence Down-Shifting: Ray-Tracing Simulations” (Progress in Photovoltaics: Research and Applications, vol. 15, pp. 27-34, 2007); and T. Maruyama and R. Kitamura in “Transformations of the wavelength of the light incident upon solar cells” (Solar Energy Materials and Solar Cells, vol. 69, pp. 207, 2001); however, no actual experiments have been performed.
Furthermore, much of the literature cautions against using photo-luminescent organic media as the stabilities of these materials are insufficient, for example see U.S. Patent Application Publication No. 2010/0012183. Most commercially available photo-luminescent media, including fluorescent dyes, exhibit photobleaching only days after solar illumination. An 11% efficiency enhancement of a CdS/CdTe cell by using Rhodamine 6G/Polyvinyl butyral film was reported by B. C. Hong and K. Kawano in “Organic dye-doped thin films for wavelength conversion and their effects on photovoltaic characteristics of CdS/CdTe solar cell” (Japan Journal of Applied Physics, vol. 43, pp. 1421-1426, 2004); however the photostability of this film was very poor under one sun (AM1.5G) irradiation. AM1.5G is a standard terrestrial solar spectral irradiance distribution as defined by the American Society for Testing and Materials (ASTM) standard 2006, see ASTM G-173-03.
According to Klampaftis et al. (Solar Energy Materials and Solar Cells 2009), only two experiments have been reported where a luminescent down-shifting material layer has been added to a Copper Indium Diselinide/Sulfide (CIS)-based cell (CIS-based devices include CIGS cells). G. C. Glaeser and U. Rau in “Improvement of photon collection in Cu(In,Ga)Se2 solar cells and modules by fluorescent frequency conversion” (Thin Solid Films, vol. 515, pp. 5964-5967, 2007) showed a 4% efficiency enhancement using a commercially available organic luminescent dye (Lumogen-F), and Muffler et al. in “Colloid attachment by ILGAR-layers: creating fluorescing layers to increase quantum efficiency of solar cells” (Solar Energy Materials and Solar Cells, vol. 90, pp. 3143-3150, 2006), reported a 3% efficiency enhancement using a quantum dot based luminescent film, however in both of these reports no data on the stability of the film was reported.